(1) Field of the Invention
This invention relates to a parabolic waveform generating apparatus used for compensating horizontal or vertical deflection distortion in television receivers.
(2) Description of the Prior Art
In the field of television receivers, Cathode Ray Tubes (CRT's) with an aspect ratio (horizontal to vertical size of the image) of (4:3) have typically been used. But recently, television receivers employing CRT's with a different aspect ratio have been developed. In a High Definition Television (HDTV) System for instance, the CRT has an aspect ratio of (16:9). In order to display an image of aspect ratio (16:9) on an ordinary CRT with an aspect ratio (4:3), several methods have been considered. One method includes a change in the vertical or horizontal deflection size.
In the parabolic waveform generating apparatus employed for compensating the horizontal or vertical deflection current in the television receivers, as well as in the other kinds of devices, digital techniques have been adopted in order to reduce the influence of change by year, in order to be able to adjust the circuits easily during production, and so on.
FIG. 1 is a block diagram of an exemplary parabolic waveform generating apparatus employing digital technique and FIG. 2 illustrates parabolic waveforms generated by the apparatus shown in FIG. 1. In FIG. 1, ROM 1, ROM 2, ROM 3, and ROM 4 are Read-Only-Memories storing the data for generating the first, second, third, and fourth parabolic waveforms respectively. And the first, second, third, and fourth parabolic waveforms are those for compensating the deflection distortion of the images with aspect ratios of (4:3), (16:9), (5:3), and (4.5:3) respectively. The selector 5 is an electronic switch which selects data for generating a parabolic waveform for the specified aspect ratio by means of a control signal.
In FIG. 2, the waveform (1) is a parabolic waveform converted in the parabolic waveform generating circuit 6 from the data signal which is read out from ROM 1 and selected by the selector 5. Therefore, the waveform (1) is a compensating current waveform for the image with an aspect ratio (16:9). Likewise the waveforms (2), (3), and (4) are parabolic waveforms converted in the parabolic waveform generating circuit 6 from the data signals which are read out from ROM 2, ROM 3, and ROM 4 respectively and selected by the selector 5. Therefore, the waveforms (2), (3), and (4) are compensating current waveforms for the images with aspect ratios of (4:3), (4.5:3), and (5:3) respectively. The same consideration can be adopted in a parabolic waveform generating apparatus for compensating horizontal deflection distortion as the above-mentioned vertical one.
However, parabolic waveform generating devices have typically had several problems. That is, multiple ROMs are typically required to store the data for generating the respective parabolic waveforms for each different aspect ratio. As more kinds of aspect ratios are requested, more ROM's are typically required.